Fine-tuning the balance between carbazole and oxadiazole units in bipolar hosts to realize highly efficient green PhOLEDs

Article abstract of DOI:10.1016/j.orgel.2013.01.010

A series of new bipolar hosts C1O1, C2O1, and C1O2 comprising hole-transporting carbazole as donor (D) and electron-transporting oxadiazole as acceptor (A) have been designed and synthesized to tune D/A ratio with a view for getting in-depth information about structure-property-performance relationship. The D/A ratio exerts subtle influences on absorption spectra and energy levels. Time-of-flight (TOF) measurements reveal that the increment of donor units effectively increase the hole mobility. The appropriate energy levels and triplet energies along with promising morphological and thermal stability of these molecules bring out them as suitable hosts to realize green PhOLEDs with (PBi)₂Ir(acac) and (PPy)₂Ir(acac) emitters. Green phosphorescent devices hosted by C1O1, C2O1, and C1O2 achieved maximum external quantum efficiencies of 20.7%, 20.4%, and 17.3%, respectively. These results indicate that carbazole/oxadiazole hybrid molecule with one carbazole and oxadiazole each is a better candidate for highly efficient green PhOLEDs.

Full text of DOI:10.1016/j.orgel.2013.01.010

Organic Electronics 14 (2013) 1086–1093
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Fine-tuning the balance between carbazole and oxadiazole
units in bipolar hosts to realize highly efficient green PhOLEDs
Shuo-Hsien Cheng a, Shu-Hua Chou a, Wen-Yi Hung b, , Hong-Wei You b, You-Ming Chen a,
⇑
Atul Chaskar a, Yi-Hung Liu a, Ken-Tsung Wong a,
⇑
a Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
b Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new bipolar hosts C1O1, C2O1, and C1O2 comprising hole-transporting carba-
zole as donor (D) and electron-transporting oxadiazole as acceptor (A) have been designed
and synthesized to tune D/A ratio with a view for getting in-depth information about struc-
ture–property–performance relationship. The D/A ratio exerts subtle influences on absorp-
tion spectra and energy levels. Time-of-flight (TOF) measurements reveal that the
increment of donor units effectively increase the hole mobility. The appropriate energy lev-
els and triplet energies along with promising morphological and thermal stability of these
molecules bring out them as suitable hosts to realize green PhOLEDs with (PBi)2Ir(acac)
and (PPy)2Ir(acac) emitters. Green phosphorescent devices hosted by C1O1, C2O1, and
C1O2 achieved maximum external quantum efficiencies of 20.7%, 20.4%, and 17.3%, respec-
tively. These results indicate that carbazole/oxadiazole hybrid molecule with one carbazole
and oxadiazole each is a better candidate for highly efficient green PhOLEDs.
Received 15 November 2012
Received in revised form 3 January 2013
Accepted 3 January 2013
Available online 8 February 2013
Keywords:
PhOLEDs
Bipolar host
Carbazole/oxadiazole
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
the amorphous thin film quality, balanced charge transport
characters to afford sufficient charge flux, high triplet en-
Phosphorescent OLEDs (PhOLEDs), known for effec-
tively utilizing both singlet and triplet excitons to achieve
a 100% internal quantum efficiency (IQE), have received
great research attentions both from academia and indus-
trial [1,2]. To realize highly efficient electrophosphores-
cence, transition metal-based emitters are usually
dispersed into a suitable host material for suppressing trip-
let–triplet annihilation and self-quenching of phosphors
[3]. As host materials essentially manage both charge
transport and recombination functions and also play a vital
role to confine emissive excitons within the emitting layer,
the molecular design of host material becomes a crucial is-
sue. Efficient host materials should possess properties such
as good morphological and thermal stability for retaining
ergy (ET) for preventing exciton quenching, and suitable
energy levels relative to the neighboring layers for giving
low operation voltage [4]. The design of bipolar host mate-
rials configured with electron-donating structures as do-
nors (D) and electron-deficient groups as acceptors (A)
seems to be the most appealing strategy in recent years
[5]. Among the hole-transporting components reported in
the literature, carbazole has found and will continue to find
extensive use in a myriad of material context for PhOLEDs
due to their good hole-transporting ability and sufficiently
high ET [6]. On the other hand, 1,3,4-oxadiazole [7] and
other electron-withdrawing groups such as phosphine
oxide [8], triazine [9], and benzimidazole [10], have been
utilized as efficient electron-transporting acceptors. Their
low-lying LUMOs are well matched to the neighboring
electron-transport layer (ETL), diminishing electron-injec-
tion barrier and effectively transporting electrons to emit-
ting layer. Due to the intrinsic dual-transport property,
hole and electron mobilities and even energy levels could
⇑
Corresponding authors. Tel.: +886 02 24622192x6718; fax: +886 02
24634360.
E-mail addresses: wenhung@mail.ntou.edu.tw (W.-Y. Hung), ken-
wong@ntu.edu.tw (K.-T. Wong).
1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2013.01.010

S.-H. Cheng et al. / Organic Electronics 14 (2013) 1086–1093
1087
be individually tuned via given different donor and accep-
2. Results and discussions
tor in one bipolar host material [11]. In addition, the phys-
ical properties of bipolar host materials and eventually the
performance of device adopting them are closely governed
by the number of hole- (HT) and electron- (ET) transport-
ing moieties and their linking topology. For example, Lee
et al. reported a series of bipolar materials comprised of
different numbers of carbazole and diphenylphosphine
oxide [12] where the HOMO and the LUMO can be suitably
tailored to align with their neighboring active layers. In
addition, the charge mobilities of bipolar molecules evi-
dently vary along with the D/A ratio. Consequently, under
the same device structure, the host material with lower D/
A ratio gave higher current density, which can be directly
attributed to the better electron transport property and
lower ETL/EML energy barrier [13]. Because the charge bal-
ance is highly related to the inherent carrier mobility of
functional materials and the interfacial charge injection
barriers, this result revealed a facile way to modulate and
balance the charge fluxes, giving an excellent example of
structure–property–performance relationship of bipolar
host materials. Considering the promising physical proper-
ties and good performance of carbazole- and oxadiazole-
containing OLED materials, we reported in this paper the
synthesis, properties and device characteristics of a series
of bipolar host materials C1O1, C2O1, and C1O2 compris-
ing of different combinations of D (carbazole) and A (oxa-
diazole). The energy levels as well as carrier mobility of
the bipolar host materials can be manipulated through
altering the D/A ratio, while maintaining sufficiently high
ET to confine emissive excitons within the emitting layer.
The devices gave the maximum external quantum effi-
2.1. Synthesis
Scheme 1 depicts the synthesis of the title compounds
C1O1, C2O1, and C1O2. Starting from the CN-substituted
carbazole derivatives 1 [14], 2 [15], and 3 [16], which were
converted to their corresponding tetrazole intermediates 4,
5, and 6 in excellent yields by treating with NaN3 and NH4-
Cl. The tetrazoles were reacted with biphenyl acid chloride
7, which was prepared from its corresponding acid [17], to
give the desired bipolar host materials C1O1, C2O1, and
C1O2 with excellent isolated yields. Their structural identi-
ties and purities were confirmed by satisfactory spectro-
scopic data and elemental analysis (see Supporting
information). The resulting bipolar molecules were config-
ured with the electron-accepting oxadiazole group attach-
ing to the C3 position of electron-donating carbazole
group. In addition, the peripheral oxadiazole was capped
by a phenyl group together with a bulky 4-tert-butylphen-
ylene at the ortho position to bring in steric hindrance and
highly twisted conformation for improving morphological
stability.
2.2. Crystal structures
The molecular structures of C1O1 and C1O2 were
unambiguously verified from single-crystal X-ray diffrac-
tion analysis (Fig. 1). The highly twisted manner between
the 4-tert-butylphenyl group and end-capping phenyl
group can be obviously observed with dihedral angles of
61.11° for C1O1 (C21–C26–C27–C32) and 73.47° for
C1O2 (C13–C18–C19–C24). Therefore, the introduction of
twisted phenyl rings as peripheral blocking groups can
enhancing the morphological stability by efficiently pre-
venting the molecules from stacking. In addition, the car-
bazole and oxadiazole moieties in these two materials
are nearly coplanar as evidenced by the small dihedral
angles of 2.31° for C1O1 (C7–C2–C1–N1) and 4.90° for
ciency (gext) of 20.7%, 20.4%, and 17.2% as C1O1, C2O1,
and C1O2 were adopted as hosts for green phosphor
(PPy)2Ir(acac), respectively. These results revealed that
bipolar host C1O1 containing equal number of HT and ET
components gave the most efficient device due to better
interfacial energy alignment and smaller charge injection
barrier.
Scheme 1. Synthetic routes of C1O1, C2O1, and C1O2.

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S.-H. Cheng et al. / Organic Electronics 14 (2013) 1086–1093
Fig. 1. ORTEP diagram of (a) C1O1 and (b) C1O2 with thermal ellipsoids shown at the 50% probability level.
C1O2 (C7–C8–C11–N2), indicating possible extended
-conjugation.
length peaks are assigned to be the
chromophores with extended -conjugation in the pres-
p–
pꢀ transition of
p
p
ence of oxadiazole substitution(s). This assumption can
be verified by the observation that the absorption maxi-
mum shifts successively with the increasing number of
oxadiazole such as C1O1 (322 nm), C2O1 (323 nm), and
C1O2 (333 nm). The PL spectra of C1O1 and C2O1 centered
around 400 nm were assigned to the intramolecular charge
transfer (ICT) emission, which was confirmed by strong
solvatochromic behavior in emission spectra (Fig. S1 in
Supporting Information). Surprisingly, the emission spec-
trum of C1O2 also centered at 400 nm, but performed rel-
atively weak solvatochromism, indicating low charge-
separated tendency in the excited state of C1O2. To shed
light on the structure–property relationship, we performed
theoretical calculations on the frontier molecular orbitals
using density function theory (DFT) with 6-31Gꢀ basis sets
(B3LYP/6-31Gꢀ) for the geometry optimization (Fig. 3). It is
interesting to note that the HOMOs of C1O1 and C2O1 are
mostly localized on the electron-donating carbazole cen-
ter, whereas the LUMOs are shifted to the peripheral elec-
tron-accepting oxadiazole moieties, leading to an obvious
spatial separation of frontier orbitals. This result fully
agrees with the strong solvatochromism observed in
C1O1. In contrast, the HOMO and LUMO of C1O2 are effec-
tively delocalized over the electron-donating and -accept-
ing moieties, leading to a weak trend of charge transfer
upon photo-excitation. These results support the observed
red-shifted absorption and the absence of prominent sol-
vatochromic effect in the emission spectra of C1O2. The
neat film PL emissions of these molecules are red-shifted
by 10–16 nm as compared to those in CH2Cl2 solution,
2.3. Thermal properties
The thermal stability and morphological property of
C1O1, C2O1, and C1O2 were determined by thermogravi-
metric analysis (TGA) and differential scanning calorimetry
(DSC), respectively (Table 1). The decomposition tempera-
tures (Td), corresponding to 5% weight loss, were ranged in
320–400 °C, and the glass-transition temperature (Tg) were
varied from 95 to 143 °C. It is clear that the thermal and
morphological stability closely correlate to the molecular
weights. The introduction of the tert-butylphenyl moiety
greatly improves the Td and Tg as compared to those of a
bipolar molecule without such substitution reported previ-
ously [18]. As a result, these relatively high Td together
with high Tg highlight the suitability of these material to
render high quality of amorphous thin film upon vacuum
thermal evaporation, which is a desirable property for
the material to be used in realization of highly efficient
PhOLED as it suppresses the aggregates formation and im-
parts morphological stability.
2.4. Physical properties
Fig. 2 presents the UV–vis absorption and photolumi-
nescence (PL) spectra of C1O1, C2O1, and C1O2 in CH2Cl2
solution. All the molecules exhibit two distinct peaks in
the absorption spectra, where the absorption peaks around
300 nm are attributed to the p–
pꢀ transition of the carba-
zole moiety [11g,19] and the neighboring longer wave-
Table 1
Physical properties of C1O1, C2O1, and C1O2.
kabs, sola (nm)
kabs, film (nm)
kPL, sola (nm)
kPL, film (nm)
ET (eV)
HOMO/LUMO/Eg (eV)
b
c
Tg/Tc/Tm (°C)
Td (°C)
C1O1
C2O1
C1O2
95/–/–
128/237/–
143/212/273
322
395
399
300, 322
292, 323
305, 333
302, 329
295, 327
307, 337
393
389
384
403
401
400
2.52
2.55
2.50
5.55/2.24/3.31
5.70/2.30/3.40
5.50/2.15/3.35
a
b
c
Measured in CH2Cl2.
Measured in EtOH at 77 K.
HOMO energy level determined by using photoelectron yield spectroscopy (AC-2). LUMO = HOMO + Eg, where Eg is calculated from the absorption
onset of film state.

S.-H. Cheng et al. / Organic Electronics 14 (2013) 1086–1093
1089
LUMO levels were calculated from the equation of
LUMO = HOMO + Eg, where Eg is the optical band gap deter-
mined from the onset wavelength of thin-film absorption
spectra. The HOMO/LUMO energy levels of C1O1, C2O1,
and C1O2 obtained by AC-2 are summarized in Table 1.
ET =2.52eV
(a) C1O1
Solu.
Film
Phos.
2.5. Carrier-transport properties
To further understand the charge-carrier transport
properties of C1O1, C2O1, and C1O2, we used the time-
of-flight (TOF) technique to evaluate the carrier mobilities
[21]. Fig. 4a–c displays representative TOF transient for
holes. The transit time, tT, can be evaluated from the inter-
section point of two asymptotes in the double-logarithmic
representation of the TOF transient. It is then used to
determine the carrier mobilities, according to the equation
ET=2.55eV
(b) C2O1
l
= d2/VtT, where d is the sample thickness and V is the ap-
plied voltage. Fig. 4d reveals that the field dependence of
the hole mobility follows the nearly universal Poole–Fren-
kel relationship, with values in the range from 2 ꢂ 10ꢁ5 to
8 ꢂ 10ꢁ7 cm2 Vꢁ1 sꢁ1. The bipolar molecule C2O1 contain-
ing one free-substituted carbazole unit exhibits much
higher hole mobility as compared to those of bipolar mol-
ecules with oxadiazole-substituted carbazole (C1O1 and
C1O2), where the hole mobilities are in the same order of
magnitude. In contrast, the transient photocurrent signals
for electrons were too weak for us to evaluate their mobil-
ities using the TOF technique. Therefore, we fabricated
electron-only devices having the device structure of ITO/
BCP (30 nm)/compounds (50 nm)/TPBI (20 nm)/LiF/Al as
shown in Fig. 5. Here, low-HOMO 2,9-dimethyl-4,7-diphe-
nyl-1,10-phenanthroline (BCP, le of 10ꢁ7 cm2 Vꢁ1 sꢁ1 or-
ET=2.50eV
(c) C1O2
300
400
500
600
700
Wavelength (nm)
Fig. 2. Room-temperature absorption and emission (PL) spectra of C1O1,
C2O1, and C1O2 in the form of solution and neat films and corresponding
phosphorescence (Phos.) spectra recorded from their EtOH solutions at
77 K.
der)
[22]
and
1,3,5-tris(N-phenylbenzimi-dazol-2-
yl)benzene (TPBI, le of 10ꢁ5 cm2 Vꢁ1sꢁ1 order) [23] as the
hole-blocking layer to impede the hole carriers in elec-
tron-only devices. In spite of the energy barriers in the
ETL/Host interface, the similar J–V curves for these elec-
tron-only devices suggest that C1O1, C2O1, and C1O2 have
comparable electron-transport abilities. Based on these re-
sults we conclude that molecules C1O1, C2O1, and C1O2
have bipolar transporting behavior.
which is typically encountered in bipolar organic materials
[20]. The triplet energies (ET) of C1O1, C2O1, and C1O2
were determined to be 2.52, 2.55, and 2.50 eV, respec-
tively, from the highest peak as the vibronic 0–0 transition
of T1 ? S0 in the phosphorescence spectra acquired at 77 K
in EtOH. Therefore, these materials should have sufficiently
high triplet energies for hosting various green phosphores-
cent emitters.
Cyclic voltammetry (CV) was performed to probe the
electrochemical property of C1O1, C2O1, and C1O2
(Fig. S2 and Table S2). As referred to the redox couple of
ferrocene/ferrocenium, the HOMO and LUMO energy levels
of C1O1, C2O1, and C1O2, derived from the onsets of ano-
dic and cathodic peaks, were estimated to be ꢁ5.74/ꢁ2.32,
ꢁ5.67/ꢁ2.28, and ꢁ5.82/ꢁ2.30 eV with respect to the vac-
uum level respectively. According to the theoretical analy-
sis, we assigned the reduction would occur on the
oxadiazole center, giving C1O1, C2O1, and C1O2 similar
reduction potentials. Whereas, the oxidation center is
localized on the carbazole core, which would be slightly af-
fected by the number of acceptor groups as indicated that
the HOMOs of C1O1 and C1O2 are ꢁ5.74 and ꢁ5.82 V,
respectively. C2O1 was found to have the lowest HOMO
of ꢁ5.67 V, which is localized on the unsubstituted carba-
zole. For a more practical use, the HOMO levels of C1O1,
C2O1, and C1O2 in thin film were determined from the
photoelectron yield spectroscopy (Riken AC-2), and the
2.6. OLED device
To investigate the new carbazole/oxadiazole hybrid
materials (C1O1, C2O1, and C1O2) as hosts in organic light
emitting diodes, we fabricated the emitting device with
typical multi-layer architecture of indium–tin–oxide
(ITO)/polyethylene dioxythiophene: polystyrene sulfonate
(PEDOT:PSS,
9,90-fluorene (DTAF, 25 nm)/emitter (25 nm)/1,3,5-tris(N-
phenylbenzimidizol-2-yl)benzene (TPBI, 50 nm)/LiF
(0.5 nm)/Al (100 nm). In view of the favorable triplet en-
ergy gaps of host (2.50–2.55 eV), we selected bis(2,N-diph-
enylbenzimidazolito)iridium(III)
(PBi)2Ir(acac) [24] and bis(2-phenylpyridinato) iridium(III)
acetylacetonate (PPy)2Ir(acac) [2b] as green phosphors
10 wt% doped into the host as emissive layer. DTAF, with
30 nm)/1,1-bis[(di-4-tolylamino)phenyl]-
acetylacetonate
a
high triplet level (ET = 2.87 eV) and wide bandgap
(Eg = 3.47 eV) can efficiently confine the triplet energy of
phosphors, is selected as the hole-transporting layer

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S.-H. Cheng et al. / Organic Electronics 14 (2013) 1086–1093
HOMO
LUMO
C1O1
C2O1
C1O2
Fig. 3. Frontier molecular orbitals (HOMO and LUMO) of C1O1, C2O1, and C1O2 calculated with DFT on a B3LYP/6-31G level.
(HTL) [25]. To further confine the holes or generated exci-
tons within the emissive region, TPBI with a high-energy
gap is selected as the electron-transporting layer (ETL)
[11e]. PEDOT:PSS and LiF served as hole- and electron-
injecting layer, respectively. Table 2 summarizes the EL
characteristics of these PhOLEDs.
Fig. 6 presents the current density–voltage–luminance
(J–V–L) characteristics, device efficiencies, and EL spectra
of the devices incorporating C1O1 (device A), C2O1 (device
B) and C1O2 (device C). The notations 1 and 2 indicate the
devices fabricated with green phosphors of (PBi)2Ir(acac)
and (PPy)2Ir(acac), respectively. These devices exhibited
turn-on voltages of 2.0 V (defined as the voltage at which
EL became detectable) and maximum brightnesses of
104,000–144,000 cd mꢁ2. In devices A1 and A2, we em-
ployed C1O1 as the host for (PBi)2Ir(acac) and (PPy)2-
Ir(acac), respectively. Device A2 had a maximum external
remained high (20%; retaining >96% of the external quan-
tum efficiency), suggesting that balanced injection and
transport of holes and electrons in the device occurred
even at high current densities and reduce efficiency roll-
off. For (PBi)2Ir(acac)-based device A1, it also exhibited
excellent device efficiencies (20.7%, 71.2 cd Aꢁ1, 71 ml Wꢁ1).
When compared with devices A1 and A2, devices B1
and B2 with C2O1 as the host achieved similar device effi-
ciencies (gext of 19.6–20.4%) as shown in Fig. 6b. However,
only the device C based on C1O2 reveals significantly lower
device efficiencies (gext of 14.5–17.3%). This is consistent
with the higher current density of C1O2-based device than
those of the corresponding C1O1- and C2O1- based devices
as shown in Fig. 6a. The energy barriers between the
HOMOs of C1O2 (ꢁ5.5 eV) and DTAF (ꢁ5.22 eV) are smal-
ler than that of C1O1 and C2O1. Therefore, holes can be
easily injected into the EML, leading to unbalance holes
and electrons in the emitting layer. In addition, the devices
A and B incorporating C1O1 and C2O1 host exhibited high-
er efficiencies relative to those of device C incorporating
C1O2 as host, which may be due to the poor hole injection
of devices A and B leading to the balanced carrier injec-
tion and transport in EMLs. We attributed this large
quantum efficiency (
current efficiency (
c) of 77.1 cd Aꢁ1. Due to the low oper-
ating voltages of device A2, its maximum power efficiency
p) reached 81 lm Wꢁ1, and such high gp is comparable
gext) of 20.7%, which corresponds to
g
(g
with green PhOLEDs lacking a p–i–n junction [26]. When
the brightness reached 1000 cd mꢁ2, the value of gext

S.-H. Cheng et al. / Organic Electronics 14 (2013) 1086–1093
1091
(a)
(c)
(b)
(d)
Fig. 4. Hole transient photocurrent signals for (a) C1O1 (1.8
l
m thick) at E = 8.3 ꢂ 105 V/cm, (b) C2O1 (2.2
l
m thick) at E = 4.1 ꢂ 105 V/cm, and (c) C1O2
(1.9
l
m thick) at E = 8 ꢂ 105 V/cm. Insets are the double logarithmic plots of (a–c). (d) Hole mobilities of C1O1, C2O1, and C1O2, plotted with respect to E1/2
.
2000
1500
1000
500
0
C1O1
C2O1
C1O2
electron-only device
2.18eV
2.7eV
2.30eV
3eV
LiF/Al
BCP
TPBI
(50 nm)
(30 nm)
(20 nm)
5.50eV
5.7eV
ITO
6.2eV
6.5eV
4
0
8
12
16
20
Voltage (V)
Fig. 5. Current density–voltage (J–V) characteristics of electron-only device.
Table 2
EL performance of devices as a function of the host and emitter.
a
Host: 10% dopant
Von (V) At 1000 nitb (V, %) Lmax (cd mꢁ2
)
Imax (mA cmꢁ2
)
gext (%) gc (cd Aꢁ1
)
gp (lm Wꢁ1
)
CIE1931 (x, y)
A1 C1O1: (PBi)2Ir(acac)
A2 C1O1: (PPy)2Ir(acac)
B1 C2O1: (PBi)2Ir(acac)
B2 C2O1: (PPy)2Ir(acac)
C1 C1O2: (PBi)2Ir(acac)
C2 C1O2: (PPy)2Ir(acac)
2
2
2
2
2
2
5.8, 20
5.7, 20
6, 19.5
6.2, 18
5.6, 16.8
5.2, 14.2
127,000, 12.5 1180
144,000, 11.5 1020
162,000, 12.5 1460
20.7
20.7
20.4
19.6
17.3
14.5
71.2
77.7
66.4
71.6
58.4
53.5
71
81
66
66.6
61.5
44.4
0.39, 0.58
0.34, 0.62
0.39, 0.58
0.34, 0.62
0.38, 0.59
0.34, 0.62
125,000, 12
1270
104,000, 11.5 1270
108,000, 11.5 1960
a
Turn-on voltage at which emission became detectable.
The values of driving voltage and gext of device at 1000 cd mꢁ2
b
.

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S.-H. Cheng et al. / Organic Electronics 14 (2013) 1086–1093
hosted by C1O1, C2O1, and C1O2, respectively. We ac-
(a)
counted the best performance of C1O1-based device to
the well matched energy levels and balanced charge flux
along with effective exciton confinement. The molecular
structure-physical property-EL performance relationship
achieved in this work clearly indicates that varying the D
to A ratio and linking topology could be a promising way
to design new bipolar host materials for the realization of
highly efficient PhOLEDs.
Host:
PBi2 /
PPy2
105
C1O1 (A1)
C2O1 (B1)
C1O2 (C1)
/ (A2)
1500
1000
500
0
/ (B2)
/ (C2)
103
101
10-1
10-3
Acknowledgements
0
2
4
6
8
10 12
We are grateful to National Science Council (NSC 100-
2112-M-019-002-MY3, 101-2119-M-002-007-MY3) and
Ministry of Economic Affairs of Taiwan for financial
support.
Voltage(V)
(b)
80
60
40
20
0
20
16
12
8
Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2013.01.010.
4
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